This invention relates in general to solvents for use in solubilizing dyes and, more particularly, to solvents used in the production of carbonless paper, microcapsules containing a chromogenic compound solubilized by such solvents, and pressure sensitive marking and recording material coated with such microcapsules.
Carbonless paper and other marking systems depend upon localized contact between a chromogenic compound such as a leuco dye and a color developing substance to produce a visible indicia. In such marking systems, the dye is solubilized in a solvent and the dye and solvent are then emulsified in an aqueous solution to form tiny "oil" droplets. The oil droplets are then encapsulated by the polymerization or coagulation of a suitable material to form a wall around the individual droplets.
In one application, the microcapsules which are formed can be coated on the back side of a sheet of paper or other material to form a coated back sheet. A second sheet of paper is then coated on the front side with a color developing substance, typically a proton donating component such an acidic clay or phenolic resin. The microcapsule outer walls isolate the dye from the developer until the microcapsules are ruptured by the application of localized pressure. When pressure is applied by a stylus, pen, typewriter, printer or similar instrument, the microcapsules burst and the oil solution containing the solubilized dye is released and is transferred to the second sheet of paper where the dye reacts with the acid component to form an image. Carbonless paper systems of this type are described in U.S. Pat. Nos. 3,418,656 and 3,418,250, which are incorporated by reference herein in their entirety.
Solvents which are utilized in carbonless paper systems should be capable of solubilizing the leuco dye, have a low water solubility, a viscosity which is low enough to permit the microcapsule contents to be rapidly transferred to the developer upon rupture of the microcapsule, a low or negligible odor, low volatility to reduce evaporation during processing, low toxicity, negligible health risks, and low cost. Various solvents have been identified which satisfy many of the above requirements.
The suitability of particular carbonless paper solvents is dependent upon the characteristics of the particular dyes and developers utilized. For example, a low viscosity solvent is generally required when phenolic acid developers are utilized in order to achieve rapid image development. By contrast, when using an acid clay developer, a higher viscosity solvent can be used with acceptable results.
The presence of diisopropylbiphenyl and triisopropyl-biphenyl as impurities in a solvent comprising primarily mono-isopropylbiphenyl was disclosed in U.S. Pat. No. 3,627,581 to Phillips. In that patent, it was disclosed that a mixture of isopropylbiphenyl positional isomers provides better fad resistance and print intensities in comparison to higher alkylated biphenyls such as butylbiphenyl, amylbiphenyl, hexylbiphenyl, decylbiphenyl, diisopropylbiphenyl, and triisopropylbiphenyl. It was further disclosed that a certain amount of the higher alkylated biphenyl impurities could be tolerated in the isopropylbiphenyl isomer mixture. However, when the impurities are polyisopropylbiphenyls, namely diisopropylbiphenyl with trace amounts of triisopropylbiphenyl, it was taught that they should comprise no more than 55% by weight of the total biaryl content of the solvent.
Alkylated biphenyls having up to 24 carbon atoms in the substituent alkyl groups were identified as carbonless paper solvents in U.K. Patent Publication No. 1,352,597. Diisopropylbiphenyl was specifically identified as a suitable solvent in that patent.
The use of diisopropylbiphenyl as a component in a carbonless paper solvent was also disclosed in U.S. Pat. No. 4,774,136 to Okada et al. In that patent it was disclosed that diisopropylbiphenyl could be used to prevent the crystallization of p-monoisopropylbiphenyl at low temperatures. The solvent mixture consisted essentially of (i) 30 to 80% by weight of p-monoisopropylbiphenyl or a biphenyl mixture of not less than 80% by weight of p-monoisopropylbiphenyl, not more than 20% by weight of m-monoisopropylbiphenyl and not more than 10% by weight of diisopropylbiphenyl, and (ii) 70 to 20% by weight of diisopropylnaphthalene or a specified naphthalene mixture.
A carbonless paper solvent comprising diisopropylbiphenyl and minor amounts of triisopropylbiphenyl is commercially available from Koch Chemical Company under the trademark Sure-Sol 330. The solvent comprises approximately 81.6% by weight of a mixture of diisopropylbiphenyl positional isomers and 2.3% by weight of a mixture of triisopropylbiphenyl positional isomers.
While the suitability of diisopropylbiphenyl as both a minor and a major component of a carbonless paper solvent has been recognized in the patents and commercial product discussed above, the use of triisopropylbiphenyl as other than a trace component of such a solvent has not previously been disclosed.